Create Alias Descriptor

Creates a new LDT data descriptor that has the same base and
limit as the specified descriptor.

Call With:

   AX     = 000AH
   BX     = selector

Returns:

   if function successful
   Carry flag= clear
   AX     = data selector (alias)

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8011H  descriptor unavailable
           8022H  invalid selector

Notes:

o  The selector supplied to the function may be either a
   data selector or an executable selector. Note that the
   published 0.9 specification was in error to say that the
   function generates an error on a data descriptor.
o  The descriptor alias returned by this function will not
   track changes to the original descriptor.  In other
   words, if an alias is created with this function, and the
   base or limit of the original segment is then changed,
   the two descriptors will no longer map the same memory.
o  Refer to the rules for descriptor usage in Appendix D.